The purpose of a thrust reverser when an airplane lands is to improve the ability of an airplane to brake by redirecting forward at least some of the thrust generated by the turbojet. In this phase, the reverser obstructs the jet pipe and directs the jet ejected from the engine toward the front of the nacelle, thereby generating a reverse thrust which adds to the braking of the wheels of the airplane.
The means employed to redirect the jet in this way vary according to the type of reverser. However, in all cases, the structure of a reverser comprises moving covers which can be moved between, on the one hand, a deployed position in which they open up within the nacelle a passage intended for the diverted jet and, on the other hand, a retracted position in which they close off this passage. These moving covers can also perform a deflecting function or may simply activate other deflecting means.
In cascade-type thrust reversers, for example, the moving covers slide along rails so that on retreating during the opening phase, they uncover the cascades of deflector vanes positioned within the thickness of the nacelle. A system of link rods connects this moving cover to locking doors which deploy into the jet pipe duct and block off the direct jet outlet. In door-type thrust reversers by contrast, each moving cover pivots in such a way as to block off the jet and deflect it and is therefore active in this redirection.
In general, the moving covers are actuated by hydraulic or pneumatic actuators which require a pressurized-fluid transport network. This pressurized fluid is conventionally obtained either by bleeding air off the turbojet in the case of a pneumatic system or by tapping off the airplane hydraulic circuit. Such systems require a great deal of maintenance because the slightest leak in the hydraulic or pneumatic network may be difficult to detect and carries the risk of having serious consequences both for the reverser and for other parts of the nacelle. Furthermore, because of the lack of space available in the front section of the reverser, fitting and protecting such a circuit are particularly tricky and space-consuming operations.
Another disadvantage with the hydraulic and pneumatic systems is that the actuators or the motor always deliver the maximum power for which they were designed and which has to correspond to the power needed to open or close the reverser under heavily loaded landing or takeoff situations. More specifically, these are, in particular, opening (or deployment) in the event of an aborted takeoff, and closure (or retraction) in the event of an aborted landing, which scenarios require a great deal more motive power than is required under normal circumstances to overcome the stresses associated with a very high turbojet speed. The issue in particular is one of being able to provide enough power that, on the one hand, during opening, the strong depression created by the direct jet which opposes the onset of opening of the moving cover and detachment of its closure seal can be overcome and, on the other hand, upon closing, the higher opposing aerodynamic forces can be overcome. Although rare, these operating scenarios have of course to be taken into consideration for safety reasons.
Because the power delivered by the actuators is always the maximum power needed to ensure that the reverser works in these heavily laden scenarios, the loads exerted on the structures and the equipments are always the highest loads, therefore leading to premature fatigue wear of the various components of the reverser. Furthermore, should a component of the reverser become jammed, the dynamic and static loads will also be very high.
For example, if the latches that latch the reverser closed become jammed, the dynamic loadings due to a shock of the moving cover in motion will be very great. The latch has to be designed to be able to withstand this shock with a motor (or a system) acting at full speed and full power. It will therefore be necessary for this latch to be overengineered to the detriment of the overall mass. This same line of arguing may apply to other components of the reverser.
More specifically, with a pneumatic or hydraulic system, the probability of a shock occurring at full power is equal to the probability of the reverser becoming jammed because full power is delivered each time it is used. According to manufacturer standards, reverser jamming is therefore an extreme case the probability of occurrence of which is too high to be able to tolerate plastic deformation of the components. The solution adopted to avoid such deformations is for the components of the thrust reverser to be overengineered at the expense of the overall mass.
It should be noted here that the mass of the equipments is an essential aspect in aeronautical design and that the thrust reverser constitutes the heaviest nacelle subassembly. It is therefore advantageous to seek to reduce this mass as far as possible while at the same time meeting safety and strength standards.
In order to offset the disadvantages associated with the pneumatic and hydraulic systems, thrust reverser manufacturers have sought to replace them and to equip their reversers as far as possible with electromechanical actuators which are lighter in weight and more reliable. A reverser such as this is described in document EP 0 843 089. However, the issue of the forces exerted on the structure has not been entirely resolved because it is still necessary for the electric motors to be capable of operating the reverser under the heavily laden scenarios.